The generation of plasma due to electrical input has been studied both experimentally and theoretically in recent years [see references 1-4]. The basic mechanisms inherent in non-equilibrium discharges such as obtained through DC, RF, or microwave excitation have also been utilized for ionization purposes, so as to increase the conductivity of air for further control with ponderomotive forces generated with an imposed magnetic field. Dielectric barrier discharge (DBD) involves one dielectric coated electrode that is typically exposed at the surface to the surrounding atmosphere, while another electrode is embedded inside a layer of insulator. The emission of UV light as well as chemical processes in surface plasmas is suitable for decontamination in a short timescale and using very low power and heat [see references 5, 6].
It has been found [see reference 5] that with special DBD arrangements, a fast reduction of cells by more than four orders of magnitude is possible within a few seconds, even for UV resistant cells. Moisan et al. [see reference 7] have observed that in contrast to classical sterilization where the survival curves of microorganisms under UV irradiation show a unique linear decay, plasma sterilization yields survival diagrams that show three basic mechanisms. First, a rapid direct destruction by plasma related UV irradiation of the genetic material of microorganisms; second, a gradual erosion of the microorganisms due to intrinsic photodesorption to form volatile compounds intrinsic to the microorganisms; and third, erosion of the microorganisms due to etching from radicals formed due to plasma ionization. Together, plasma sterilization is much (order of magnitude) faster than the traditional sterilization process.
Traditionally, in plasma discharge, a DC voltage potential is placed across two electrodes. If the voltage potential is gradually increased, at the breakdown voltage VB, the current and the amount of excitation of the neutral gas becomes large enough to produce a visible plasma. According to Paschen's law, the breakdown voltage for a particular gas depends on the product (p×d) of the gas pressure and the distance between the electrodes. For any gas there is unique p×d value referred to as the Stoletow point where volumetric ionization is the maximum. The Stoletow point for air requires a minimum VB=360 V and p× d=5.7 Torr-mm.
Unfortunately near atmospheric pressure, the allowable electrode spacing necessary for maximum volumetric ionization is d=7.7 μm. In some applications, for example in high-speed air vehicles, this is an impractical limitation. A solution to this limitation comes from the recent development of RF glow discharge using an a.c. voltage potential across the electrodes. The frequency of the current must be such that within a period of the a.c. cycle, electrons must travel to the electrodes and generate a charge, while the heavier ions cannot. Based on reported experiments [see reference 2] in air or other gases at 760±25 torr, a homogeneous glow can be maintained at 3 to 20 kHz RF and rms electrode voltage between 2 to 15 kV. A critical criterion for such discharge in air is to meet the electric field requirement of about 30 kV/cm. While the voltage is high, only a few milliamps current is required to sustain a RF driven barrier discharge.